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Last update : 2015/09/21
In this article, I propose a solution to photograph shooting stars automatically.
This solution is based on a detection principle of shooting stars known for over 40 years.
The amator and professional astronomers used this principle to count the shooting stars.
The principle is relatively simple: a meteor or shooting star that ionizes the air meet, the ionized air
reflects radio waves, it is sufficient to capture this, to trigger the digital SLR.
To understand the principle, we must do a bit of theory on the airwaves.
The AM airwaves (short airwaves, small airwaves, and big airwaves) are reflected by the atmospheric layers.
That's why, when we are between two mountains, we continue to receive music from the radio station.
The first radio stations, broadcast in AM airwaves, with big powers, we could receive anywhere
emissions and music, even by car, even in difficult places.
The amator radio use shortwave radio to make contact with the world
and thanks to the reflection of waves on the atmospheric layers.
Then in the 80s, the FM has emerged.
It is better sound quality for music that AM,
but the reception distance is much smaller because the FM radio waves are not reflected on
the atmospheric layers, they go into space and because the frequency is higher.
That is why in car, the reception of an FM station varies
especially if you're in the mountains for example.
A small design principle is available on Photo01.
As a workaround FM reception, we multiplied the number of FM transmitter. The current car radios
automatically change the frequency (if FM transmitter change) when the reception becomes weak.
During passage through the atmosphere, a meteor or shooting star, there is an air ionization.
The FM radio waves, instead of spreading into space, bounce off the ionized air and therefore
very briefly, we can receive FM radio (photo02).
The principle is to listen to the FM radio band to capture a radio show, that in normal
you can't receive. This principle is simple but far more difficult to implement.
I will explain the technique.
To activate the digital SLR when switching from a shooting star,
we will use my "lightning detector."
This tiny electronic circuit is an interface to do everything for your DSLR
(Detect lightning, detection of a laser barrier laser, detecting a sound, ...).
This electronic assembly would be called the "interface for DSLR" but his first
was used to detect lightning so he kept the name.
Watch this site and the "Lightning Detector" for more details for its manufacture and use.
Must therefore the "Lightning Detector" and a radio FM (photo03).
I strongly advise you use an FM radio with digital view because you have to adjust the radio receiver
on an FM frequency that does not get in normal times and don't listen.
The detection of shooting star will not be possible in city or close to a FM transmitter.
you will choose a remote area of ??the cities to reduce light pollution and also
the pollution FM radios which are very numerous.
There are two ways to set up the detection of meteors:
- Either use a radio using the integred antenna. In this case you should choose a place
in a exclusion zone of FM reception. For example, a hill or a mountain as a barrier
between you and the FM transmitter (see Photo01). This method is not very adapted for plains,
but can still work if you're in the limit of reception.
- Or use an FM radio with an external antenna like a Yagi antenna.
I give the plan to build a Yagi antenna on photo05. The Yagi antenna must be positioned
on a pole about 6 feet above the ground at an angle of 10-30 degrees towards the sky from the ground.
To connect the antenna YAGI FM reception to your receptor, you will use the antenna input via a cable 75 Ohms
or disassemble the radio and connect to the place of the antenna as the photo04.
This solution is the best and can receive distant transmitters.
As the distance between you and the issuer will be larger,
one can detect a little more ahead a shooting star.
There are two major issues that must be addressed.
The frequency to set the radio?
What radio station to choose according to your position and direction of the shooting stars?
It is strongly advised to choose a low frequency between 88 MHz and 92 MHz for the majority of shooting stars,
by ionization of the air, the frequencies between 50 and 100 Mhz works.
Frequencies between 88 and 92 MHz are much less used frequencies between than 98 and 102 Mhz.
If you use a Yagi antenna, it will be necessary to calculate this for the 90 Mhz,
it will be suitable for frequencies between 88 and 92 Mhz.
The radio station or listening frequency will depend on the direction of the shooting stars and your position.
Example: During the Geminids that occur in December, the shooting stars provide from the east.
We must therefore select an FM radio station, who the FM transmitter is located on the east of your position
at a distance of 50 kms or 100 Kms (see a greater distance if the transmitter is very powerful).
The goal is to set the FM radio receiver on a channel you can not hear,
because of the distance or because of natural obstacles
(You will hear just the noise radio, Pffffffff but no music).
During the passage of the meteor, FM radio airwaves will bounce off the air ionized by the meteor.
Here is a list of transmitters powers, and distance coverage of the issuer:
A transmitter power of 30 W distance covered between 3 and 8 KM
A transmitter power of 100 W distance covered between 5 and 15 KM
Transmitter power of 300 W distance covered between 8 and 25 KM
Transmitter power of 500 W distance covered between 20 and 50 KM
Transmitter with a power of 1000 W distance covered between 50 and 80 KM
Transmitter with a power of 3000 W distance covered between 70 and 120 KM
Transmitter power of 10,000 W Distance covered between 100 and 250 KM
As you can see, this technique will allow you to detect the passage of a shooting star
and thus trigger the DSLR but it isn't possible to know the direction,
and the life of the shooting star.
If you listen to an FM transmitter to the east but the shooting star come from the west,
it will be detected, but probably it is too late and it is already passed over your head.
If the meteor is too small, it may have a very short life
and therefore do not travel the skies over a large distance.
If the meteor enters in the atmosphere above your head,
it will probably not be detected if it goes in the wrong direction.
This detector of shooting star does not capture at once the pictures of shooting stars,
but will increase your chances.
To further increase the chances of capturing a beautiful photograph,
you have to use a wide angle lens (eg 10 mm, 17 mm, 18 mm).
Your digital SLR camera should be set in manual mode,
setting example: ISO 200, F 3.5, speed 30 seconds, focus in manual mode with focus on infinity.
This sample set will allow you to photograph the shooting star but also to photograph
few stars in the sky as the photo06.
For more information:
The time of airwave propagation between the FM transmitter and FM receiver FM is negligible
because the waves travel at the speed of light.
A shooting star is moving at a speed of 20-70 km per second.
The electronic assembly (the lightning detector) takes a few milliseconds to trigger the digital SLR.
Depending on the digital SLR, the time to open the shutter varies in 50 and 150 ms (milliseconds).
So after a detection, the shooting star will be traveled, before to shoot it, a distance of:
Maximum: (70 / (1/0.150)) = 10.5 km (in the sky)
Minimum: (20 / (1/0.05)) = 1 km (in the sky)
We can therefore consider that, from the time of the shooting star is detected
and opening the shutter of the camera, the meteor has traveled an average of 5 to 10 degrees in the sky.
I hope I have provided enough explanation to detect and photograph the meteors.
For more details on the detection of meteors with an FM radio,
You can search on the Internet: "detect meteor"
There are several websites that detail the technique very well.
Also look for "Software meteor" is software that is made to count the shooting stars.
Good pictures of shooting stars ...
Copyright 2011 ThierryD - http://rienquepourlesyeux.free.fr
Last update 05/09/2011